A conventional bubble level includes a bent glass tube filled with water or alcohol having an air bubble formed in the bent tube, by which the air bubble once floats to a central highest position as defined by two graduations, it may judge a plane overlain by the bubble level to be a horizontal plane or not. However, the air bubble may quickly "run" in the bent glass tube to thereby delay the finding out of a real horizontal plane. The simple two graduations formed on a central highest position of the bent glass seem to be a rough scale, unsuitable for a precision measurement of an absolute horizontal plane.
A conventional plumb bob may be used for checking a vertical line by hanging a plumb weight on a line. However, once blown by wind or shaken by an external force, the line and plumb may swing to influence the measurement of a vertical line, especially when the vertical line is expected immediately.
The present inventor has found the defects of a conventional level and plumb bob, and invented the present simply-constructed measuring instrument.